Recently, spatial light modulators such as digital micromirror devices (DMD) for modulating illumination light by high-speed switching of micromirrors of pixels arranged in a matrix between angles corresponding to ON and OFF states according to pulse width modulation (PWM) driving based on image data have been used in small projectors.
Unlike a known liquid crystal display device, such a spatial light modulator is capable of high-speed operation, and is thus capable of displaying red (R), green (G), and blue (B) images in a frame sequential method. A projector using a liquid crystal display device requires three liquid crystal display (LCD) devices for displaying a color image, whereas a projector using a spatial light modulator is capable of color display with one DMD device.
In such a known projector using a spatial light modulator, a white light lamp has been used as a light source. For example, with a known projector using a spatial light modulator, an input image is converted into a frame-sequential image signal and supplied to the spatial light modulator; a color wheel colored in RGB is rotated in synchronization with a vertical sync signal of the input image; and the spatial light modulator is irradiated with light from the lamp via the color wheel. However, when a lamp is used as a light source of the projector, the power consumption becomes high and a color wheel is required.
On the other hand, recently, use of a LED as a light source of such a projector has been considered. An LED, compared with a lamp, has advantages such as a small size, high durability, long life, and low power consumption. By using three LEDs of RGB, a color wheel is not required, and excellent color characteristics can be achieved. Moreover, when a spatial light modulator is used, an optical system having a low-loss light source, such as an LED, generating non-polarized light can be easily constructed since, unlike a liquid crystal display device, such a spatial light modulator has no polarization dependency.
However, when an LED is driven with a direct current, there is a limit to the amount of electric current that can be applied to the LED. Thus, as described in, for example, Patent Document 1, the use of a rotating optical system has been proposed.
This rotating optical system includes a supporting member that supports a light source unit including a plurality of red, green, and blue LEDs arranged around the circumference and an optical system rod that rotates around the center axis about which the light source unit is arranged. In this rotating optical system, the plurality of LEDs is sequentially pulse-driven and the plurality of LEDs is sequentially pulse-illuminated. Then, the optical system rod is rotated around the axis in synchronization with the illumination of the LEDs, and light from the illuminated LEDs is collected and emitted towards the spatial light modulator.
Although there is a limit to the amount of electric current that can be applied when the LEDs are driven with a direct current, when a rotating optical system is used, the LEDs are pulse-driven, as described above. Therefore, a large current can be applied to the LEDs, thus achieving intense light emission. Furthermore, by using such a rotating optical system, light from the illuminated LEDs can be collected by the optical system rod, and light equivalent to that obtained when the LEDs are continuously illuminated can thus be obtained.
Patent Document 1: Japanese Unexamined Patent Application, Publication No. 2003-346503